Screw Modifier Darker Shading on Half of Object
I think the issue may be that your normals are flipped: In 3d each face has a 'normal' that represents which way the lighting should be interpreted. To fix this, in edit mode press CTRL + N. This will try to flip the normals so they point outwards. If this does not fix it, open your toolshelf and look to the Operators menu at the base of it (Looks like this). Check the box saying 'Make normals consistent inside'.